1. Field of the Invention
The invention relates to a technique for preventing radio wave interference between a plurality of radars.
2. Description of the Related Art
A vehicle (mobile unit) equipped with a radar at the front or rear thereof is practically used. The radar is able to detect an obstacle, and acquires a distance to the obstacle and a relative velocity of the obstacle. With the above configuration, the vehicle is able to avoid a collision with the obstacle or warn a driver of the collision. A known waveform of a radio wave output from such a radar, for example, includes an FM-CW mode of a frequency modulation pattern in which the frequency is increased or decreased along a triangular wave and a CW mode in which a plurality of (for example, two) frequencies are switched in a time-sharing manner (see Japanese Patent Application Publication No. 2007-187632 (JP-A-2007-187632), Japanese Patent Application Publication No. 2002-14159 (JP-A-2002-14159), Japanese Patent Application Publication No. 2004-69693 (JP-A-2004-69693), Japanese Patent Application Publication No. 2006-242818 (JP-A-2006-242818) and Japanese Patent Application Publication No. 2007-155551 (JP-A-2007-155551)). It is generally known that a radar system, when using a CW mode, is not able to detect a vehicle (an example of an obstacle) when a relative velocity with respect to the vehicle is zero. In the existing art, one radar is switched between the CW mode and the FM-CW mode (JP-A-2004-69693).
In addition, such a radar has a problem in radio wave interference with another vehicle, or the like (see JP-A-2007-187632, JP-A-2002-14159, JP-A-2004-69693, JP-A-2006-242818 and JP-A-2007-155551). JP-A-2007-187632 and JP-A-2007-155551 describe that a radar outputs a radio wave in a time-sharing manner or in a time period during which a radar of another vehicle is not outputting a radio wave. JP-A-2002-14159 describes that an FM-CW transmission wave spectrally diffused in a random coding sequence is transmitted, a correlation process is performed at a reception side, and, when a coding sequence differs from the transmitted coding sequence, radio wave interference is prevented. JP-A-2006-242818 describes that, when spike noise is output because of radio wave interference, an average value of values adjacent to the spike noise is output.
However, when one and the same mobile unit is equipped with a plurality of radars, there is a possibility that a radio wave transmitted from one radar (for example, a radar provided at the right front of a vehicle) interferes with a radio wave transmitted from another radar (for example, a radar provided at the left front of the vehicle). In order to prevent such interference, when a technique for preventing the interference of radio waves of radars in a time-sharing manner as described in JP-A-2007-187632 or JP-A-2007-155551 is applied, a radar does not transmit a radio wave while another radar is transmitting a radio wave. This elongates a time interval for detection, and there is a possibility that detection of an obstacle delays.